1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor integrated circuit device comprising a field effect transistor (FET) and a negative differential resistance element, such as a resonant tunneling hot electron transistor (RHET) and a resonant tunneling bipolar transistor (RBT), which are monolithically integrated, and a method of producing the same.
Recently, much research has been made into semiconductor devices provided with novel functions by using heterojunctions of compound semiconductor layers, especially, a resonant tunneling barrier structure. The negative differential resistance elements have the functions described below, and further development of the circuits including these elements is expected.
2. Description of the Related Art
FIG. 1 is a schematic sectional view of an RHET, as one example of the differential negative resistance elements. The RHET comprises a semiinsulating GaAs substrate 21, the semiconductor layers 22 to 26 as shown in Table 1, an emitter electrode 27, a base electrode 28, and a collector electrode 29.
TABLE 1 ______________________________________ Impurity Semiconductor Dosage Thickness Layer Composition (cm.sup.-3) (nm) ______________________________________ 22 Collector GaAs n - 1 .times. 10.sup.18 300-500 23 Collector Al.sub.0.2 Ga.sub.0.8 As Undoped 100-300 Barrier 24 Base GaAs n - 1 .times. 10.sup.18 20-100 25 Emitter Barrier 25a Al.sub.0.3 Ga.sub.0.7 As Undoped 3-5 25b GaAs Undoped 3-5 25c Al.sub.0.3 Ga.sub.0.7 As Undoped 3-5 26 Emitter GaAs n - 1 .times. 10.sup.18 .perspectiveto.400 ______________________________________
The emitter barrier layer 25 between the emitter and base of the RHET has a resonant tunneling barrier structure having a quantum well, in which the GaAs well layer 25b is sandwiched between the AlGaAs barrier layers 25a and 25c, and electron energy levels in the quantum well correspond to the discrete values, i.e., resonant levels.
The RHET of the negative differential resistance element is operated by providing the base 24 and the collector 22 with potentials positive to the emitter 26. As shown in an energy band diagram of the RHET of FIG. 1B, when the electron energy level in the emitter 26 corresponds to the resonant level E.sub.1, namely, when a base voltage V.sub.BE between the emitter 26 and the base 24 is about 2E.sub.1 /q (q is an electric charge of the electron) at a peak voltage (V.sub.P), the electrons (hot electrons) are injected from the emitter 26 into the base 24 by a resonant tunneling effect. Accordingly, the electrons pass through the base 24 at an ultra-high speed, go across the collector barrier 23, and then reach the collector 22.
When the base voltage V.sub.BE is smaller or larger than 2E.sub.1 /q, it does not meet the conditions for producing resonance tunneling, and an amount of electrons injected into the base 24, i.e., the collector current, is decreased. Therefore, as shown in FIG. 2 in which an example of an emitter ground collector current measured at a temperature of 77.degree. K. is shown, the collector current has a peak value at a certain value (V.sub.P) of the base voltage V.sub.BE.
In consequence, an inverter gate using the RHET shows the differential negative resistance characteristics whereby an output is decreased, has a lowest value, and thereafter is increased together with the increase in an input voltage V.sub.BE. For example, a 2-input exclusive-NOR gate having the following logic mechanism can be constituted with one RHET and three resistance elements, as shown in FIG. 3, although 7 or 8 elements are necessary if transistors are used.
______________________________________ A B C ______________________________________ 1 1 1 0 0 1 0 1 0 1 0 0 ______________________________________
In addition to the above-mentioned RHET, the negative differential resistance element may be, for example, an RBT having an n-p-n bipolar transistor structure in which the base layer is p-type. The use of these negative differential resistance elements enables functional effects such as a decrease in the number of elements in a logic circuit, a curtailment of time lags, and the like to be obtained. Therefore, where a desired logic circuit is constituted, it is necessary to monolithically integrated a transistor (e.g. FET) having a usual switching mechanism with the differential negative resistance element.
However, the differential negative resistance element is entirely different from an FET in the constitution of a semiconductor multilayer structure, and therefore, the constitution of the semiconductor multilayer structure and the method of producing same are problems in the process of monolithic integration.